Bioventing

The following description of bioventing is an excerpt from Chapter III of OUST's publication: How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites: A Guide for Corrective Action Plan Reviewers. (EPA 510-B-95-007). This publication also describes 9 additional alternative technologies for remediation of petroleum releases. You can download PDF files of every chapter of the document at: http://www.epa.gov/swerust1/pubs/tums.htm.

Bioventing is an in situ remediation
technology that uses indigenous
microorganisms to biodegrade organic
constituents adsorbed to soils in the
unsaturated zone. Soils in the capillary fringe
and the saturated zone are not affected. In
bioventing, the activity of the indigenous
bacteria is enhanced by inducing air (or
oxygen) flow into the unsaturated zone (using
extraction or injection wells) and, if
necessary, by adding nutrients.

When extraction wells are used for
bioventing, the process is similar to soil
vapor extraction (SVE). However, while SVE
removes constituents primarily through
volatilization, bioventing systems promote
biodegradation of constituents and minimize
volatilization (generally by using lower air
flow rates than for SVE
). In practice, some degree of volatilization and biodegradation
occurs when either SVE
or bioventing is used.

Application

All aerobically biodegradable constituents
can be treated by bioventing. In particular,
bioventing has proven to be very effective in
remediating releases of petroleum products
including gasoline, jet fuels, kerosene, and
diesel fuel. Bioventing is most often used at
sites with mid-weight petroleum products
(i.e., diesel fuel and jet fuel), because lighter
products (i.e., gasoline) tend to volatilize
readily and can be removed more rapidly
using SVE
. Heavier products (e.g., lubricating
oils) generally take longer to biodegrade than
the lighter products.

Bioventing is not appropriate for sites with groundwater tables located less than 3
feet below the land surface. Special considerations must be taken for sites with a
groundwater table located less than 10 feet below the land surface because groundwater
upwelling can occur within bioventing wells under vacuum pressures, potentially
occluding screens and reducing or eliminating vacuum-induced soil vapor flow. This
potential problem is not encountered if injection wells are used instead of extraction
wells to induce air flow.

If a cleanup level lower than 0.1 ppm is required for any individual constituent or a reduction in TPH greater than 95 percent is required to reach the cleanup level for TPH, either a pilot study should be required to demonstrate the ability of bioventing to achieve these reductions at the site or another technology should be considered.

Operation Principles

Soil normally contains large numbers of diverse microorganisms including bacteria,
algae, fungi, protozoa, and actinomycetes. In well-aerated soils, which are most appropriate for bioventing, these organisms are generally aerobic. Of these organisms, the bacteria are the most numerous and biochemically active group, particularly at low oxygen levels. Bacteria require a carbon source for cell growth and an energy source to sustain metabolic functions required for growth. Nutrients, including nitrogen and phosphorus, are also required for cell growth. Hydrocarbon-degrading aerobic bacteria use oxygen to metabolize organic material to yield carbon dioxide and water, a process commonly referred to as aerobic respiration. To degrade large amounts of petroleum hydrocarbons, a substantial bacterial population is required which, in turn, requires oxygen for both the metabolic process and the growth of the bacterial mass itself. Approximately 3 to 3.5 pounds of oxygen are needed to degrade one pound of petroleum product.

Bioventing differs from SVEin one fundamental way: the objective is to induce only sufficient airflow to enhance natural biodegradation of the contaminants, not cause them to volatilize. Airflow may be induced by either extracting soil air or injecting atmospheric air. Because of the lower airflow required to achieve bioventing, there is less liklihood than with SVE of causing contaminants to be forced into areas where they could potentially cause problems (e.g., vapor accumulation in basements). For extraction systems, there is probably less of a need for vapor treatment than for SVE systems.

The most important factors that control the effectiveness of bioventing are:

The permeability of the petroleum-contaminated soils. This will determine the rate at
which oxygen can be supplied to the hydrocarbon-degrading microorganisms found
in the subsurface.

The biodegradability of the petroleum constituents. This will determine both the rate
at which and the degree to which the constituents will be metabolized by
microorganisms.

In general, the type of soil will determine its permeability. Fine-grained soils (e.g.,
clays and silts) have lower permeabilities than coarse-grained soils (e.g., sands and
gravels). The biodegradability of a petroleum product constituent is a measure of its
ability to be metabolized by hydrocarbon-degrading bacteria that produce carbon dioxide
and water as byproducts of microbial respiration. Petroleum products are generally
biodegradable regardless of their molecular weight, as long as indigenous
microorganisms have an adequate supply of oxygen and nutrients. For heavier
constituents (which are less volatile and less soluble than many lighter components),
biodegradation will exceed volatilization as the primary removal mechanism, even
though biodegradation is generally slower for heavier constituents than for lighter
constituents.

Note that the ability of a soil to transmit air, which is of prime importance to bioventing, is reduced by the presence of soil water, which can block the soil pores and reduce air flow. This is especially important in fine-grained soils, which tend to retain water.

Soil structure and stratification are important to bioventing because they affect how
and where soil vapors will flow within the soil matrix when extracted or injected.
Structural characteristics such as microfracturing can result in higher permeabilities than
expected for certain soils (e.g., clays). Increased flow will occur in the fractured but not
in the unfractured media. Stratification of soils with different permeabilities can
dramatically increase the lateral flow of soil vapors in more permeable strata while
reducing the soil vapor flow through less permeable strata. This preferential flow
behavior can lead to ineffective or extended remedial times for less-permeable strata or
to the possible spreading of contamination if injection wells are used.

System Design

In general, remedial approaches that rely on biological processes should be subject to field pilot studies to
verify and quantify the potential effectiveness of the approach and provide data
necessary to design the system. For bioventing, these studies may range in scope and
complexity from a simple soil column test or microbial count to field respirometry tests
and soil vapor extraction (or injection) pilot studies. The scope of pilot testing or
laboratory studies should be commensurate with the size of the area to be remediated,
the reduction in constituent concentration required, and the results of the initial
effectiveness screening.

Design Radius of Influence (ROI) is an estimate of the maximum distance from a
vapor extraction well (or injection well) at which sufficient air flow can be induced to
sustain acceptable degradation rates. Establishing the design ROI is not a trivial task
because it depends on many factors including intrinsic permeability of the soil, soil
chemistry, moisture content, and desired remediation time. The ROI should usually
be determined through field pilot studies but can be estimated from air flow modeling
or other empirical methods. Generally, the design ROI can range from 5 feet (for
fine-grained soils) to 100 feet (for coarse-grained soils). For sites with stratified
geology, radii of influence should be defined for each soil type. The ROI is important
in determining the appropriate number and spacing of extraction or injection wells.
Stratified soils may require special consideration in design to ensure that less-permeable
strata are adequately vented.

At a site with homogeneous soil conditions, the well should be screened throughout the contaminated zone. The well screen may be placed as deep as the seasonal low water table. A deep well helps to ensure remediation of the greatest amount of soil during seasonal low groundwater conditions.

At a site with stratified soils or lithology, the screened interval can be placed at a
depth corresponding to a zone of lower permeability. This placement will help ensure
that air passes through this zone rather than merely flow through adjacent zones of
higher permeability.

Airflow is particularly important for soils within the capillary fringe, where a
significant portion of the constituents often reside. Fine-grained soils create a thicker
capillary fringe than coarse-grained soils. The thickness of the capillary fringe can
usually be determined from soil boring logs (i.e., in the capillary fringe, soils are usually
described as moist or wet). The capillary fringe usually extends from one to several feet
above the elevation of the groundwater table. Moisture content of soils within the
capillary fringe may be too high for effective bioventing. Depression of the water table
by groundwater pumping may be necessary to biovent soils within the capillary fringe.

Fluctuations in the groundwater table should also be considered. Significant seasonal or daily (e.g., tidal or precipitation-related) fluctuations may, at times, submerge some of the contaminated soil or a portion of the well screen, making it unavailable for air flow. These fluctuations are most important for horizontal wells, in which screens are placed parallel with the water table surface and a water table rise
could occlude the entire length of screen.

Bacteria require moist soil conditions for proper growth. Excessive soil moisture,
however, reduces the availability of oxygen, which is also necessary for bacterial
metabolic processes, by restricting the flow of air through soil pores. The ideal range for
soil moisture is between 40 and 85 percent of the water-holding capacity of the soil.
Generally, soils saturated with water prohibit air flow and oxygen delivery to bacteria,
while dry soils lack the moisture necessary for bacterial growth. Bioventing promotes dehydration of
moist soils through increased air flow through the soil, but excessive dehydration hinders
bioventing performance and extends operation time.

The optimum pH for bacterial growth is approximately 7; the acceptable range for
soil pH in bioventing is between 6 and 8. Soils with pH values outside this range prior to
bioventing will require pH adjustments prior to and during bioventing operations.

Bacteria require inorganic nutrients such as ammonium and phosphate to support cell
growth and sustain biodegradation processes. Nutrients may be available in sufficient
quantities in the site soils but, more frequently, nutrients need to be added to soils to
maintain bacterial populations.

The presence of very high concentrations of petroleum organics or heavy metals in
site soils can be toxic or inhibit the growth and reproduction of bacteria responsible for
biodegradation. In addition, very low concentrations of organic material will also result
in diminished levels of bacterial activity.

Advantages:

Uses readily available equipment; easy to install.

Creates minimal disturbance to site operations. Can be used to address inaccessible areas (e.g., under buildings).

Requires short treatment times: usually 6 months to 2 years under optimal conditions.

Cost competitive: $45-140/ton of contaminated soil.

Easily combinable with other technologies (e.g., air sparging, groundwater extraction).

May not require costly offgas treatment.

Disadvantages:

High constituent concentrations may initially be toxic to microorganisms.